The enduring lesson of 1914 is that people are not predestined to mutual slaughter. ''There are always choices,'' Margaret MacMillan concludes in her magnificent book, The War That Ended Peace.

In beautifully measured prose MacMillan narrates the story of why World War I happened and the accumulation of warnings that might have prevented it. The Great War was not racially ''necessary'' as decreed by the bastardised doctrine of ''social Darwinism'' widely believed at the time, as MacMillan shows. Nor were the guns of August somehow ''inevitable'', as the leaders argued in their post-war memoirs. All later claimed they'd fought a defensive war for which they were innocent of responsibility.

MacMillan's point ought to be engraved on their tombs: in August 1914, the European powers chose war. They variously willed or acquiesced in the decision. Their positions ranged from the extreme bellicosity of the Austrian Chief of Staff, Count Conrad von Hotzendorf, to the lame vacillations of British foreign secretary, Sir Edward Grey. Wilful or complacent, the effect was the same: a conscious descent into the worst conflict the world had known.

The Great Powers had, however, shown they were capable of peaceful resolutions: they had defused the Balkan Wars (1912-1913), for example, and the colonial tensions in Morocco. England and Germany had weathered the navy race. So why were they helpless to resolve the crisis of 1914 peacefully? Mediation and containment were scarcely applied. Why? MacMillan suggests that the confluence of hatred, paranoia, war planning, the arms race and sheer fatalistic despair reached a critical mass that overwhelmed the weak men in charge. Her inescapable point is that the Great Powers decided for war. Nor was Germany solely to blame, as recent histories have contended: all the powers had a hand in the destruction of Europe.

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Let us remember why many still feel a sense of burning indignation at this tragedy, and why the war's apologists ought to hang their heads in shame: the Great War killed or wounded 37 million people, tore up the map of Europe and left a deeply dysfunctional residue: Germany humiliated, Russia in the grip of Bolshevism, and Britain and France wretched and broken. The best part of a generation of young men had been wiped out. And then it led to a second, more devastating conflict. The mind recoils from dreaming of how Europe might have looked, had it not happened.

One of MacMillan's recurring themes is that the political leaders were simply not up to the job. Many were unintelligent, utterly unversed in statecraft and promoted above their ability. Her portraits of the bumbling politicians and bewhiskered generals who ran the world make a superb case for the prosecution against inherited power (though her flippant, if apt, likening of the Kaiser to Toad of Toad Hall may not impress her Oxford colleagues).

There is, however, a significant flaw in this book: the epilogue. Having psychologically primed the reader for a tough-minded conclusion,MacMillan's hitherto decisive and penetrating voice seems to desert her. She resorts to asking rhetorical questions, about blame and responsibility. She lists what became of the Kaiser, the Tsar, the politicians and commanders, as if she were wrapping up the fortunes of the stars in a film. Where was the profound lament, the marshalling of her arguments, the rapier thrusts at the gross irresponsibility of the Powers? Perhaps the author was, understandably, exhausted?

She draws some odd conclusions, which the previous 590 pages do not seem to support. For example: ''What is surprising about the Great War is … that Russia, Germany and Austria-Hungary endured for so long before they collapsed into revolution or mutiny or despair.'' On the contrary, they were fighting to preserve their ailing empires and dynastic systems, to uphold the divine right of kings.

Having shown us the key cause of the war - all too human failure - MacMillan seems to retreat into her academic igloo. At a deeper level, she implicitly questions the whole concept of historical determinism - that unseen laws predestine the species towards war or revolution - a notion the second half of the 20th century has utterly discredited. Yet we hear barely a whisper of this in her epilogue. I should confess an interest: the conclusions of my own book are in almost perfect accord with those of MacMillan's. I hope she might strengthen her closing chapter for a later edition, and deliver some decisive judgments. If so, I'd unreservedly recommend this book as of one of most incisive and brilliant narratives of the causes of the greatest tragedy of the 20th century - the greatest tragedy, because it led to all the rest.

Paul Ham is the author of 1914: The Year the World Ended. Margaret MacMillan will speak at Sydney University on March 6.